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One of the most common questions an oph-thalmologist gets asked at dinner parties isthe one about laser eye surgery. There areseveral reasons for this. Firstly, a lot ofpeople wear glasses so the news of a medical

breakthrough to save them from this task is a hot topic.Secondly, “laser eye surgery” can be purchased on thehigh street and is not currently available on the NHS.People want reassurance from a knowledgeable andhopefully neutral health professional about safety issuesand prognosis. The usual answer to people’s questions is ajumble of pros and cons, culminating in the retort “Sowhy do you wear glasses then?”

Further confusion arises when people report that theirgrandmother had laser eye surgery for her cataracts, theirdiabetic brother had NHS laser eye surgery, and an unclehad laser treatment for age related macular degeneration,which turned him yellow. Pressures on the general doctor

are increasing to have an understanding of the role oflaser technology in the management of eye conditions.This in turn will reassure patients and not contribute tofurther confusion.

This article aims to clarify what types of “laser eye sur-gery” are available in ophthalmology and aid the doctorbombarded by a barrage of questions in the surgery or,indeed, over dinner.

Historical aspectsLight has been known to affect the eye for thousands ofyears, but the key to using it for therapeutic purposes hascome relatively recently. In ancient times staring at thesun was known to permanently damage central vision andproduce central scotomata (blind areas). These resultantlesions could be studied only with the development ofinstruments to examine the retina, such as the ophthal-moscope. With this knowledge, several investigators havetried to reproduce these effects on the retina by usingfocused sunlight. In 1949, Meyer-Schwickerath publishedhis work with light photocoagulation and its effects on theretina using solar radiation.w1 The impracticalities of usingthe sun as a source of energy led to the development ofthe xenon arc photocoagulator in 1956. A xenon arc bulbwas pulsed with a current to cause a flash of white andinfrared light. This provided a more focussed and usablesource. Laser technology progressed from these begin-nings in 1960 when Maiman produced what was thencalled the “optical maser” using a ruby crystal source.w2

This produced a “pure” beam of light at a wavelength of694.3 nm that could provide much smaller burns with avariable intensity. Over time, lasers were further devel-oped using different sources to produce accurate andreproducible light beams and used in the treatment of awide range of ocular disorders.

Laser—the science bitThe term laser is an abbreviation for “light amplificationby stimulated emission of radiation.” In the most com-monly used ophthalmic lasers, a powerful electric currentis passed through a plasma tube containing one of severalgases (argon, krypton) or a solid material (neodymiumYAG) known as the laser medium or source. The result isexcitation of the atoms of the laser medium in the plasmatube (population inversion) and the production of burstsof light or photons (spontaneous emission). These pho-tons continue to strike the atoms in the plasma tube toproduce a narrow uniform monochromatic light beam(stimulated emission).

Laser beam effects on biological tissue include heatcoagulation (a thermal effect), cutting (a photochemicaleffect), or a dissolving of tissue (an ionising effect).

Ophthalmic lasers are usually named accordingto the lasing medium. For example, the laser that

Laser eye surgery has been a subject of focus inOphthalmology in the last few years and patientsincreasingly want to know about its use in eye conditions,as Jaheed Khan and Victor Chong explain

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refractive error. Changes to the corneal curvature occurby sculpting the middle layer of the cornea or stroma. Thesuperficial layer or corneal epithelium therefore needs tobe lifted (surface ablation) before this is done by one ofthe following methods.� Manually and left to heal (photorefractive keratec-

tomy or PRK)w3

� Using an alcohol solution to soften the epitheliumbefore creating a flap to lift it and replace it after (laserassisted sub-epithelial keratomileusis or LASEK)w4

� Fashioning a flap with a microkeratome metal blade(Laser In Situ Keratomileusis or LASIK)w5

� Creating a flap by using the excimer laser (intralase).w6

The results are profound, allowing certain patients tostop wearing glasses or contact lenses. The few complica-tions that arise are mostly due to inadequate healing ofthe flap once it is replaced. This can lead to visual aberra-tions (such as glare, ghosting, or haloes), dry eye, or infec-tion. This type of laser eye surgery is currently notroutinely available on the NHS but the National Instituteof Health and Clinical Excellence (NICE) is currentlyreviewing its use.

In patients with superficial corneal scarring diseases(such as corneal epithelial dystrophy) the excimer lasercan also be used to accurately remove tissue in a proce-dure known as phototherapeutic keratectomy (PTK).w7

Diabetic retinopathy and retinal diseases: thegreen laserLaser beams with wavelengths in the blue-green lightrange have been of most use in the treatment of chorio-retinal diseases.w8 w9 The most common application forgreen laser is diabetic retinopathy. It has been successfullyused to treat diabetic maculopathy and proliferative dia-betic retinopathy.w10 w11

The retinal pigment epithelium (with its pigment gran-ules) below the neural retina is the main layer of cells thatabsorbs laser light of this wavelength. The vitreous, lensand cornea, and neural retina absorb very little energyfrom the blue-green light spectrum. This targeting of theretinal pigment epithelium cells has been useful intreating diabetic macular oedema, where exudative

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uses the gas argon and emits a green or blue-green lightbeam, is referred to as the “argon laser,” whereas the laserwhich uses the neodium YAG solid medium is referred toas the “YAG laser.” The excimer and the carbon dioxidelasers are examples of more recent developments. Theexcimer uses ultraviolet laser light to treat refractive errorby sculpting the cornea. The CO2 laser is used for cos-metic eye surgery and tear duct surgery. There are otherophthalmic lasers such as the holmium laser, the erbiumlaser, and the tunable dye laser, all with various uses.

Naming the laser after the material used in the plasmatube can be confusing, as some lasers can be tuned to pro-duce various wavelengths of laser light. For example, thefrequency of the YAG laser can be doubled, to produce agreen light beam at the same wavelength that is producedby the argon laser.

Lasers can be classified according to the lasing mediumthey use, wavelength of light they create, or colour of lightbeam they produce.

The table shows examples of commonly used lasers inophthalmology.

Ophthalmic lasers

Refractive error correction and corneal disease:the excimer laserThe excimer laser is an argon-fluoride laser that emits pulsesof ultraviolet light. The term excimer comes from the con-cept of an energised molecule with two identical compo-nents or excited dimer (contracted to one word). Each pulseof this “cool-laser removes 1/4000 mm of tissue from thetargeted surface by breaking intra molecular bonds in colla-gen molecules. It would take about 200 pulses from anexcimer laser to cut a human hair in half. This laser wasoriginally developed for use in the microprocessor industryand later found its application in vision correction.

Focussing or refractive errors such as short sightedness(myopia), long sightedness (hyperopia) and astigmatism(irregular curvature of the cornea) have traditionally beencorrected using glasses or contact lenses. The excimerlaser provides the ability accurately to sculpt or change thecurvature of the cornea permanently, resulting in achange of focusing power and a correction of the original

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a wavelength of 689 nm for about 90 seconds. The non-thermal laser light activates the dye, producing an activeform of oxygen that both coagulates and reduces thegrowth of abnormal blood vessels. This in turn inhibits theleakage of fluid from the choroidal neovascular mem-brane. Membrane lesions that arise in myopic patients canalso be treated by photodynamic therapy.w21

Diode laser also has the benefit of penetration throughscleral tissue. This has been most commonly used indiode laser cyclo-ablation or cyclodiode where red laser isapplied near the edge of the cornea and used to photoco-agulate and destroy the underlying ciliary body. Thisreduces the production of aqueous humour within the eyereducing intra-ocular pressure. This type of treatment ismost commonly used in glaucoma cases resistant to con-ventional treatment.w22 w23

Angle closure glaucoma and posterior capsuleopacification removal: the Q switched Nd YAGlaserThe ability of laser beams to dissolve tissue or photodis-rupt came about with the development of Q switching.This modification, most commonly seen in theneodymium YAG laser, results in the shortening of thepulse of the laser and boosting the peak output power tocreate a very high energy beam with the ability to ionisetissue through micro-explosions. Its most common use inophthalmology is in the prevention and treatment ofangle closure glaucoma by creating a hole in the periph-eral iris (iridotomy). This allows aqueous to flow throughand increase the size of the aqueous drainage angle.w24

Another use is in the removal of an opaque membranethat can grow over the posterior surface of certain intraoc-ular lenses (posterior capsule opacification) after cataractsurgery.w25 This helps improve light transmission throughthe lens to reach the retina and improves visual acuity.

ConclusionsLasers have been used successfully in ophthalmology formany reasons. The ability of lasers to deliver precise targetedtreatment without surgical excision into an organ such asthe eye has made treatment of certain eye diseases possible.

Also, ophthalmologists are able to directly visualisetreated areas and are able to perform repeated treatmentswith minimal complications. Techniques introduced 30years ago are still used routinely, testament to their effi-cacy. Laser technology is constantly developing andremains a vital tool in ophthalmology practice.

Jaheed Khan, Retinal Research Unit, Ophthalmology, Kings CollegeHospital, London SE5 9RSjaheedkhan@yahoo.co.ukVictor Chong clinical research fellow consultant ophthalmologist, RetinalResearch Unit, Ophthalmology, Kings College Hospital, London SE5 9RS

Competing interests: None declared.

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fluid from leaking blood vessels collects within the centralretina. By some yet unknown mechanism, stimulating theretinal pigment epithelium cells by gentle laser photoco-agulation in this thickened macula results in resolution ofthe retinal oedema. Blue light has the disadvantage ofbeing absorbed by xanthophyll pigment (a carotenoidsubstance essential for visual processing) in the macularregion and causing inner neural retinal damage whenused to photocoagulate. It has also been implicated incausing damage to the surgeon delivering thetreatment.w12 The laser wavelength most commonly usedin the treatment of diabetic macular oedema has there-fore been in the green spectrum range. Large randomisedcontrolled trials to investigate the benefits of laser photo-coagulation for diabetic macula oedema have also usedgreen laser.w13

Increasing the power of the green laser to burn thechorioretinal layers as opposed to gentle stimulation hasbeen of greatest use in the treatment of proliferative dia-betic retinopathy.w14 The destruction of peripheral retinain this stage of diabetic retinopathy leads to the regressionof the abnormal new vessels through an unknown mech-anism. Laser burns can total several thousand to achievethis destruction. Side effects of treatment include reducedperipheral vision and poor night vision, which are sacri-ficed to preserve central vision.

Other less common conditions that require green lasertreatment for macular oedema include branch retinal veinocclusionw15 and central serous retinopathy.w16 Conditionswhich require focal retinal destruction include the sealingof retinal tears that may otherwise lead to retinal detach-mentw17 and the destruction of subretinal choroidal neo-vascular membranes away from the foveal region.w18 In thetreatment of primary open angle glaucoma, green lasercan be applied to the trabecular meshwork (laser trabecu-loplasty) to facilitate aqueous outflow and reduce intra-ocular pressure.w19

Age related macular degeneration andglaucoma: the diode laser (near infrared andinfrared)Red and infrared laser light has the benefits of deeperpenetration within the retina, targeting the retinal pig-ment epithelium and choroidal vasculature. It has there-fore been used to treat lesions that are associated withthese areas. The development of a choroidal neovascularmembrane in age related macular degeneration has beendescribed as exudative or “wet.” A recent form of treat-ment for exudative age related macular degeneration isphotodynamic therapy.w20 In this therapy, a photosensitivedye known as Visudyne (verteporfin) is administeredintravenously and allowed to perfuse the choroidal neo-vascular membrane, as well as the remainder of the body,for about 15 minutes. The ophthalmologist then treats themembrane with a diode that produces a red light beam at

References and furtherinformation onthis subject are onstudentbmj.com

Laser name Laser medium Wavelength (nm) Colour of beam Uses

Excimer Argon-fluoride gas 193 Ultraviolet Refractive surgeryw3-w6

Corneal diseasew7

Argon Argon gas 488-514 Blue-green Proliferative diabetic retinopathy14

Frequency Neodymium YAG 532 Green Macular oedema secondary to diabetesw10w13

doubled YAG crystal Branch retinal vein occlusionw15

Central serous retinopathyw16

Retinal tearsw17

Extrafoveal choroidal neovascular membranesw18

Trabeculoplasty in glaucomaw19

Diode Semiconductor 689 - 810 Red Photodynamic therapy for choroidal neovascular material (silicon, membranes secondary to age related macular germanium, degenerationw20, myopiaw21

selenium) Glaucomaw22 w23

Q switched Neodymium YAG 1064 Near infrared Peripheral iridotomy for angle closure glaucomaw24

YAG crystal Posterior capsule opacificationw25

Commonly used lasers in ophthalmology